High density magnetic recording can be attained with a perpendicular magnetic recording system, as an alternative to a conventional longitudinal magnetic recording system. In this regard, a crystalline film of a CoCr alloy system has been mainly contemplated for a magnetic recording layer of a perpendicular magnetic recording medium. In perpendicular magnetic recording, the crystal orientation of the recording layer is controlled so that the c-axis of the CoCr alloy system having a hcp structure aligns perpendicular to the film surface (i.e., the c-plane is parallel to the film surface). To obtain a higher density in the CoCr alloy system, finer grain size, reduction of dispersion of grain size distribution, and decrease in magnetic interaction between grains have been contemplated.
A conventional metallic magnetic film of a CoCr system is deposited on a substrate at a high temperature. A grain boundary phase is formed by segregating chromium from the magnetic grains of the cobalt system. The grain boundary phase reduces magnetic interaction between magnetic grains. When this method is employed, the substrate needs to be heated to at least 200° C. before laminating or depositing the layers on the substrate.
A method of controlling the magnetic recording layer structure to raise the recording density in a longitudinal magnetic recording medium has been proposed, for example, in Japanese Unexamined Patent Application Publication No. H8-255342 and U.S. Pat. No. 5,679,473. These references disclose a magnetic recording layer having a structure generally referred to as a granular magnetic recording layer, where each of its magnetic grains is surrounded by a nonmagnetic, nonmetallic substance, such as an oxide or nitride. Because the nonmagnetic and nonmetallic grain boundary phase physically separates the magnetic grain in the granular magnetic film, the magnetic interaction between the magnetic grains decreases to suppress the formation of a zigzag shaped magnetic domain wall that would be formed in a transition region of a recording bit. Thus, low noise can be achieved. It is known in such a granular magnetic recording layer that the interaction between magnetic grains can be reduced even if substrate heating is not performed. This is because the nonmagnetic, nonmetallic substance used for the grain boundary phase segregates easier than conventionally used chromium, and thus can be segregated without heating.
In the same vein, a granular magnetic recording layer is contemplated for a recording layer of a perpendicular magnetic recording medium combining the above-described techniques in IEEE Trans. Mag., Vol. 36, p2393 (2000). Specifically, this publication discloses a perpendicular recording medium that comprises a ruthenium metal underlayer and a magnetic recording layer of a CoPtCrO alloy having a granular structure. The c-axis alignment enhances with increased thickness of the ruthenium underlayer for the granular magnetic recording layer. The thickness of the ruthenium underlayer, however, needs to be about 40 nm or more to obtain satisfactory magnetic characteristics and electromagnetic conversion performance in the granular perpendicular magnetic recording medium. Since ruthenium is an expensive rare metal element, it is desirable to reduce the thickness of the ruthenium underlayer to reduce the production cost.
A so-called double layer perpendicular magnetic recording medium has been proposed, where a soft magnetic backing layer is included under the magnetic recording layer to enhance the sharpness of the magnetic field generated at a head region. To enhance recording density of such a magnetic recording medium, the thickness of the nonmagnetic layer disposed between the magnetic recording layer and the soft magnetic backing layer needs to be no greater than 20 nm. Consequently, the above-described structure having a thick ruthenium underlayer poses technological restriction to producing a double layer perpendicular magnetic recording medium.
To attain a higher recording density, noise has to be reduced. One of the factors that deteriorate performance of a perpendicular recording medium is increase in dispersion of orientation distribution in the magnetic recording layer. The axis of easy magnetization of the magnetic recording layer in a perpendicular recording medium needs to be aligned perpendicular to the surface of the medium. If the dispersion of orientation distribution of the axis of easy magnetization is large, the output signal decreases due to decrease of the perpendicular component of the magnetic flux. In addition, the transition looses sharpness and noises increase. Therefore, the dispersion of orientation distribution in the magnetic recording layer needs to be reduced as small as possible to achieve high output and low noise in a perpendicular magnetic recording medium.
In addition to reducing the dispersion of orientation distribution, the initial growth layer of a magnetic recording layer needs to be decreased to improve the performance of a perpendicular recording medium. A magnetic recording layer of a conventional perpendicular magnetic recording medium includes an initial growth layer of several nm having an in-plane component of magnetization. The initial growth layer is a source of noise. This is considered to be caused by an inferior alignment of the underlayer of the titanium alloy used in a conventional perpendicular magnetic recording medium or interdiffusion between the underlayer and the magnetic recording layer. From the view point of recording, a thin magnetic recording layer is desirable. However, if the initial growth layer exists and the film thickness is decreased, the relative proportion occupied by the initial growth layer increases, decreasing the S/N ratio (SNR). That is, the initial growth layer inhibits reduction of the thickness of a magnetic recording layer.
In addition to the above, noise reduction of a magnetic recording medium requires decrease in the grain size of the magnetic recording layer. When the grain size of the magnetic recording layer increases, a bit transition region becomes zigzag shaped and the transition noise increase. Accordingly, reduction of the transition noises requires a smaller grain size and the bit transition region needs to be strait line shape.
Accordingly, to improve the performance of a perpendicular magnetic recording medium, the dispersion of orientation distribution in the magnetic recording layer needs to be decreased, the initial growth layer in the magnetic recording layer needs to be reduced, and the grain size of the magnetic recording layer needs to be reduced. The present invention addresses ways of improving the performance of a perpendicular magnetic recording medium.